The invention relates to a catalyst, a catalyst system, an arrangement for recombining hydrogen with oxygen, and a method for producing a catalyst.
Hydrogen is used in large quantities in numerous industrial processes. The main concern when handling hydrogen is the need to avoid ignitable gas mixtures. Mixtures of hydrogen and air are ignitable within a very broad range of concentrations between 4 and 75 vol. % hydrogen. The spontaneous ignition temperature for a hydrogen/air mixture is about 560° C. While the occurrence of ignitable mixtures is avoided during normal technical operations, such mixtures also occur outside of the processes as a consequence of leaks or operational malfunctions. However, in nuclear reactors, hydrogen can also be formed on the order of magnitude of 10,000 standard cubic meters when there are malfunctions involving a core meltdown. With the air oxygen present in the containment vessel, the hydrogen can form an ignitable mixture that represents a threat to the integrity of the containment vessel.
Due to statutory and legal considerations, especially in a nuclear reactor, in case of damage, the exchange of atmospheres must be prevented and the ignitable mixture must be diluted to a safe hydrogen concentration. So-called recombiners are therefore used as safety precautions. They make the mixture leaner in that the hydrogen is converted to water vapor in an exothermal reaction with the air oxygen. These recombiners can contain a catalyst material so that hydrogen and oxygen also react with one another in a safe manner below the ignition temperature.
Due to the enormous heat from recombination, approximately 242 kJ/mol, the recombiner can overheat and thus become the ignition source itself. Therefore, recombiners are frequently used only below the ignition limit of 4 vol. % hydrogen or their performance is limited by installing diffusion barriers. Examples of this are known from K. Ledjeff, “Elimination of hydrogen or oxygen from explosive mixtures by catalytic techniques,” International Journal of Hydrogen Energy 1987, Volume 12, No. 5, pages 361-367, and from DE 199 14 814 C1. However, because of this, very explosive and therefore hazardous mixtures remain in the containment vessel for a very long time.
Known from DE 197 22 305 and DE 198 52 953 are technically further developed recombiners having passive heat absorbing and storing elements and cooling systems that are intended to counteract the overheating problem. However, since these elements and systems are not designed for every conceivable case, in these cases, as well, it is not possible to preclude the possibility of overheating with potentially catastrophic consequences.
In addition, a recombiner can be exposed to the ambient atmosphere for very long standing times until its potential employment. During this standing time, disadvantageous contaminants from the environment can contaminate the catalyst so that recombination does not initiate completely without a substantial delay. Recombiners must undergo regular maintenance because of this.
The object of the invention is therefore to provide means with which hydrogen and oxygen can be recombined, it being ensured that the temperature will not under any circumstances exceed a prespecified limiting temperature, regardless of the concentration and homogeneity of the hydrogen. These means should also provide their full capability immediately, even after lengthy standing times.